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ceplialic margin, they are 8 milliiii. in lengtli, the caudal margin measuring 5.5 millim. Springing out immediately below the 

 angle between the lower jaw and the ventral surface of the body, they first take a directly ventral direction, and slightly 

 medial, and then the distal, triangular part inclines in a medial and caudal dii'ection. The cephalic margin forms a convex 

 curve with a slight concavity on the transition between the antibraehium and the manus; the caudal margin is broken into 

 a short, concave, proximal part (fig. 10), which forms an obtuse angle with the slightly longer and concave, distal part. The 

 cephalic and caudal margins converge distally so as to form an acute angle. The distal part, or manus of the pectorals, is 

 longer than the proximal part, which corresponds to the fore and upper arm. Four decided rays, or digits can be distinguished, 

 the longest of which lies in the cephalic margin. 



In the middle of the arched surface of the abdomen is seen a distinct raphe running from the navel forwards 

 towards the lower jaw, and backwards towards the tail. Surrounding the insertion of the navel is an oval field, more pointed 

 in front, and bounded on each side by a slight prominence. This oval umbilical field denotes that part of the wall of the 

 abdomen which is covered by the membrana reuniens anterior, while the outer boundary indicates the medially growing 

 side-plates. If we compare this embryo with the one immediately before it, we see here a further advance of the side 

 plates. Below the navel, the l)oundaries of the umbilical field are confluent at the outer genitalia (PL II, fig. 11). 



At this stage the genital aperture is distinctly separated from the anus. The merabrum genitale is large and 

 projecting, and bends towards the tail. like a hook. Farthest in on the under side, it is hollowed out, and the genital opening 

 lies like a cavity at its base. Immediately below it, the anus is observable in the form of a small dot-like opening. 



On both sides of tlie genital aperture, a somewhat long, whitish, and slightly raised region can be distinctly seen, 

 denoting the rudiments of the mammæ pudendal es (Pl. II, fig. 11). 



The almost cylindrical, conical cauda shows signs of side compression, as its ventral and dorsal sides present a 

 sharp edge. Towards the end, the side surfaces become prominent, forming the commencement of the side lobes of the tail 

 fin (PI. II, fig. 11). Neither in the preceding stadium nor in this one have I found anything to indicate that "nearly the 

 whole of the free tail becomes broader by lateral folds of skin", as Kiikenthal states (cf. Lit. No. 58, II Theil. pag. 263). 

 There is still a distinct bend in the tail, although its distal pa]t is turned more in the direction of the long axis of the 

 body. It is, however, beginning to incline towards the side (right). 



The umbilical cord is 18 millim. long, and 4 millim. broad at the place of insertion. It increases a little in 

 thickness immediately outside the insertion, and then gradually decreases towards the periphery. 



Although the colour (fig. 10) in this hardened specimen is a uniform grey, a skin-colouring can clearly be seen 

 in the neck region. This at first appears like a "'neck-band", extending sideways towards the eye, and backwards in the 

 median line of the back. It is not. however, until later that this colouring exhibits its characteristic distribution. 



IV. When the embryo, measured in a straight line from the crown of the head to the extended end of the tail, 

 attains a length of from 7 to about 10 centim.. several famlbj characters already begin to appear in addition to the fætal 

 Cetacean ones. While the head gradually takes a more foreward direction, so that the nasal aperture becomes the most 

 advanced point, instead of the crown of the head as in earlier stages, the commencement of the dorsal fin is also visible, the 

 little tail lobes become more distinct, and the pectoral fins assume more and more their peculiar, pointed, elegant shape 

 (Taf. II, figs. 12—14). 



Of the foetal curves in the smallest (74 millim.') long) of the <6 foetuses of this stadium at our disposal, the head 

 fiexion is still apparent; in the longer embryos the facial region has become a little more straightened out (cf. fig. 12). The 

 nose is longer and more pointed than in the 45 millim. long embryo. In the 74 millim. long specimen, the production of the 

 line of the mouth cleft forms a somewhat obtuse angle with the long axis of the body ; in the slightly longer embryo this 

 angle, which is open to the surface of the abdomen, becomes more obtuse (figs. 12, 13). At the side of the snout region of 

 the larger embryos, a slight indication of a break may be detected, a characteristic mark of the Linnæan Delphinus genus. 

 On the upper lip there is a distinct raised line with from 5 to 7 dot-like prominences denoting the commencement of the 

 fætal hairs (PI. 11, fig. 12). The mouth cleft extends so far backwards, that the frontal plane through the angle of the mouth 

 will intersect the nasal apertures. The obliquely-set eye cleft forms an acute angle with the mouth cleft, if the latter be 

 imagined produced backwards until it intersects the hind edge of the eye cle!t. The region of the eye itself forms a hemispherical 

 protuberance at the sides. 



The nasal orifice, in the about 90 millim. long embryo, forms a slight concavity behind, with its edges a little 

 raised. In the smaller embryos there is a small prominence in the middle of the nasal cleft, most marked on the hind edge, 

 but this becomes more obliterated in tlie larger embrvos. 



') Measured iVom the .'iid of the nose ti) tlie audit. irv passage, and thence to the end of the tail, the length is 77 millim. 



3 



